San Diego Gas & Electric is betting that a startup company with an untested technology to generate solar power can provide it with much of the renewable energy it will need to meet a state mandate by 2010.

The plan is at the heart of SDG&E's arguments that the Sunrise Powerlink, a proposed 1,000-megawatt line across the desert and mountains, is needed to bring renewable energy from the Imperial Valley.

Meanwhile, the company behind the technology says it might not build its plant if Sunrise isn't approved because it won't have a way to sell its power.

Those issues and others will be on the table Thursday, when the California Public Utilities Commission is expected to decide whether Sunrise gets built and whether it should put any conditions on the line's construction.

For most people, the idea of solar power brings to mind flat photovoltaic cells that convert sunlight into electricity. This project is different. It would rely on the sun's heat to run thousands of engines, which would drive generators to create electricity.

The engine that Stirling Energy Systems of Phoenix is counting on for its contract with SDG&E was invented in 1816 by a Scottish minister, Robert Stirling, as a safer alternative to the steam engine.

Other companies have adapted it to run on heat from the sun, but none has made it work on this scale.

Stirling proposes to build 30,000 mirrored dishes, each 40 feet wide and 38 feet tall, to focus sunlight onto engines smaller than a lawn mower's, heating parts of them to more than 1,300 degrees Fahrenheit.

Inside the sealed engine, hydrogen would expand under that intense heat and drive a piston, then move to a shaded area where, cooled by desert air, it would contract and drive another piston.

The engine in each dish would produce about 25,000 watts. Together, thousands of them covering three square miles in the Imperial Valley would produce more power than a large natural-gas-fired plant.

Stirling has six prototype engines mounted on dishes and producing energy at Sandia National Laboratories in Albuquerque, N.M., where engineers are working on modifications to increase the amount of solar energy they convert to electricity.

The effort appears to be working. Popular Mechanics honored the project in October for converting 31 percent of the solar energy that hit its mirrors into electricity – more than any other system has ever done.

But 12-year-old Stirling has had trouble turning the lab work into commercial success.

“There's been a lot of words, but there has not been a lot of action,” said Steven Cowman, who took over as chief executive of Stirling in April, when Irish conglomerate NTR bought a controlling stake from initial investors for $100 million. “We're trying to fix that.”

NTR is planning to put another $100 million into Stirling in the next year, and will reverse the problems it faced before it was funded, when promises to vendors and suppliers were not kept, Cowman said.

Stirling plans to have Linamar, a Canadian automotive products company, produce its first engine in mid-January, test it, then enter full production by the end of next year.